If you own one blue it is probably ultramarine, and the first time you buy phthalo blue to go with it you will make a mess. Not because either is a bad paint, but because they differ in two ways at once: which way they lean, and how hard they push.

Lean

Ultramarine (PB29) is a warm blue: it leans towards violet. Phthalo blue green shade (PB15:3) is a cool blue: it leans towards green. This decides what each is good for. Mix ultramarine with a yellow and the violet in it fights the yellow, so the green comes out muted, which is often exactly what a landscape wants. Mix phthalo with the same yellow and the green is clean and bright. Reverse the exercise with alizarin: ultramarine gives a clean violet, phthalo gives a dull one.

Strength

Phthalo blue has about twice the tinting strength of ultramarine in Halftint's data (2.2× against 1.1× the average tube). To reach the same pale sky tint, the engine needs about 7 parts of white for one part of ultramarine and about 14 parts for one part of phthalo. Put another way, a knife of phthalo is worth two knives of ultramarine in a mix, and a pile of it will absorb white for a long time before it looks pale.

Side by side

Four blues, mixed with Halftint's engine: tinted with white at 1:4 and 1:12, mixed with cadmium yellow light at 1:2, and mixed with alizarin crimson at 2:1. Cobalt and cerulean are included because they are the other two blues a painter is likely to be weighing.

BlueStrengthTintsGreenViolet
  • Ultramarine Blue
 1.1×
  • 1:4 white
  • 1:12 white
  • 1:2 cad. yellow light
  • 2:1 alizarin
  • Phthalo Blue (GS)
 2.2×
  • 1:4 white
  • 1:12 white
  • 1:2 cad. yellow light
  • 2:1 alizarin
  • Cobalt Blue
 0.9×
  • 1:4 white
  • 1:12 white
  • 1:2 cad. yellow light
  • 2:1 alizarin
  • Cerulean Blue
 0.9×
  • 1:4 white
  • 1:12 white
  • 1:2 cad. yellow light
  • 2:1 alizarin

Read across the green column: cerulean and cobalt make the yellowest, cleanest greens, phthalo a deep clean green, ultramarine a muted one. Read across the violet column: ultramarine makes the only violet worth the name. The tints show the strength: at 1:4 the phthalo tint is still a definite blue where the others have gone pale.

Which to buy

  • Only one blue: ultramarine. It makes the violets, the greys with burnt sienna, and the muted greens most landscapes need, and it is forgiving in quantity.
  • Adding a second: phthalo if you paint water, glass, clear skies or tropical greens and want saturation; cerulean or cobalt if you want a cool blue that is easier to control and mixes softer greens.
  • Handling phthalo: it goes into a mix last, a speck at a time. Start from the pile of white or yellow and touch the phthalo in. The mixing-order guide explains why this is not fussiness.

Halftint flags phthalo blue and every other strong tinter on the recipe card and puts it last in the order automatically. If you want to see what your own blue can and cannot mix, load it in the app and look at the wheel.